Mechanics of MS brake regeneration

I was wondering if someone here understands the mechanics behind the brake regeneration system on the Tesla. I also have a hybrid ICE which uses brake regeneration as well. When I brake lightly, it shows me it is using brake regen to charge the battery (not using the brake pads at all). If I press on the brake harder, it simply regenerates the battery faster. If I press the brake pedal hard enough, the actual brake pads of the car kick in. This system seems very efficient and intelligent.

In speaking with reps about my MS, it seems it doesn't work that way: When you let off the gas, aggressive regeneration kicks in automatically. But if you press on the brake pedal(even a little bit), it doesn't increase regeneration but instead activates the actual brake pads.

So my questions are:
1. Do I have that correct with the differences between my hybrid car and MS?
2. If true, why would Tesla design it that way? My hybrid system makes much more sense: it uses less battery and regenerates more energy. It would work even better in the Tesla because you wouldn't need to "adjust" how aggressive you want regen to be. It would simply increase regen as you increasingly apply the brakes until it reaches the maximum and then would automatically transition to the brake pads.

Also, new MS drivers wouldn't have to "get used to" the aggressive regeneration...


1) Yes, you are correct in so much as regen is controlled soley by the "go" pedal. The brake pedal has no part in the regen system

2) I'm assuming you do not have your Tesla S yet. I have had mine for 5 months. After a few minuites of driving, it's very natural. The S is designed to be operated more or less with one foot. And that makes even much more sense!

You are correct in how the MS regeneratively brakes. However I disagree with your assessment that the MS system is inferior. The MS does require a little more time to master but it becomes second nature very quickly. The thing I hated about pushing the brake to activate regen was that I never knew when the brake pads started to engage. In my MS if I want to slow quickly I just let off the accelerator. If that's not enough, it usually is, then I apply the brake. I hardly ever use the brake pedal.
I disagree that it makes more sense. There are many times when you need to brake a little harder or a little faster than the regen default. That is when a lot of energy is lost to the brake pads that could have gone to the battery.

Would you at least concede that the brake pedal should also tie into the brake regen system and transfer that excess energy to the battery before using the brake pads??

In my hybrid, there is a large gauge that shows you how much is going to the battery as you are braking and it kicks on the brake pads when it reaches the limit of the gauge. This is what Lexus told me anyways...

The regen isn't aggressive if you just just ease off the accellerator. The regen varies by how much you ease off.

In practice, it means you end up using a single pedal for most of your driving. In normal urban driving, you can speed up quickly and then slow down as you approach a light or traffic all with one pedal. In highway driving, you just modulate your position in traffic with the single pedal.

It all feels quite natural, and the result is that you are always in control of the car and driving it. In other cars, when you lift off the accelerator, you are coasting at a fixed drag rate (the ICE engine causing most of the drag through energy losses in the pistons and tranmission), but you are not able to control the exact speed or relative traffic location of the car. You also flip from accelerator to brake and back again frequently.

I suggest you test drive a Model S. You'll find that it is very fun way to drive. I describe it as having the downshift and clutch modulation ability of a manual tranmission, except with one pedal, no noise and a heck of a lot more energy efficiency.

The regen does not have to be on full. Lifting the goose pedal partway gives part regen. You'll get the hang of it quickly.

Also, when you do use the brake, that doesn't mean the regen turns off. The regen is still occurring at the full amount consistent with the speed of the vehicle. The brakes just add more stopping power.

@shop +1

Test drive it.

Another reason not to tie regen to the brake pedal at all is safety. The more complex your brake system, the more opportunities for failure. Brakes are better off being dirt-simple IMO; push brake pedal, engage pads.

@bronto - the MS optimizes regen.

Simply lift your foot off the go pedal to begin regen. When your foot is off the go pedal, you are getting max regen.

Whenever your foot is on the brake (assuming you drive with one foot), you are always getting max regen.

Works much better than my Prius, which regens when I press lightly on the brake. I wish there were an indicator so I would know how hard to press to get max regen, without using the brake pads at all.

The MS has the added benefit that if you need to hit the brakes in a hurry, you get the speed reduction of regen even before you foot gets over to the brake pedal. This will shorten the "real world" emergency stopping distance - and with the average age of an MS driver seeming to be around 50, need all the help we can get to our slowing reflexes...


do you even know how much regen you are getting with your inferior regen? model s is capable of putting back 60KW of energy. also it isn't a mechanical system so less to wear out.

model s has the best regen system hands down. the hybrid you are driving can't even put this efficient regen in due to technical reasons.

the getting use to you are talking about is no more than a day of driving and you will wonder why every car isn't like this. only pure electric drive can be done this way,

sorry, tesla designed it this way because they were looking to design the best car. you think you are smarter than the people who do this for a living?

@bronto, If you haven't yet got it, think of it this way. As you let off the GO peddle your regen will increase until it maxes out the system and can't increase any more. With your foot off the peddle the MS is putting all the regen Tesla Motors feels comfortable back into the battery. At this level the car is actually breaking fairly hard. The MS is a very heavy vehicle so it takes more energy to accelerate and decelerate from a given speed than most other vehicles.

What I like about the Tesla regen model is that when you need to brake in a hurry, before your foot has even reached the brake pedal, you are already braking.
Moving regen over to the brake pedal as in the hybrid model effectively increases your braking distance

Caveat: I'm basing this in reports, videos, and some conjecture. I've not yet had the chance to drive one, never mind have to perform an emergency stop in one :'(

IMHO, the primary reason they do it this way is for safety -- you leave the friction brakes alone, and they are a completely separate, well-tested system. When I was considering building a conversion EV, I was going to do exactly the same things because I didn't want to make the brakes drive-by-wire and I didn't have the facilities to do the testing required to be confident in that.

If you really believe that "one pedal driving" is the reason, then why wouldn't you move the friction brakes to it as well? Ie, as you lift your foot off the accelerator, you brake with both regen and friction, and if you want to coast or brake lightly you simply keep light pressure on the accelerator.

The other advantage of the Tesla approach is better brake feel--its easier to modulate and control the brakes because they behave like a traditional system. The hybrid braking systems I have used on test drives always feel mushy and disconnected to me. If you read the auto mag reviews, you will also see similar comments.


One other factor that may be a consideration is the case when the battery is full.
With the hybrid approach, the point where the physical brakes are used needs to vary

@jat - you would not move the friction brakes to the single pedal because if your foot slipped off the pedal you would come to a screaching halt with full brakes.

I have a Prius and a MS. Agree with what was said above. The MS approach makes more sense because it enables one-foot driving (which makes rolling hills feel effortless) and gives you complete control on when you want to regen and when you want to brake. I also feel keeping the brake system independent makes it more reliable and predictable. It does take some getting used to (especially when switching back and forth) but I prefer MS approach by far.

@tes-s - so then it isn't single pedal driving.

@kribensa - yes, but since you are already doing it with software, you can adjust that automatically. With the Tesla approach, you leave it up to the driver to react to apply friction brakes when you don't have much regen braking available.

If anything, the Tesla way is smarter, because it winds up being more efficient. It does the same thing in the last few centimeters of the accelerator's travel when you release it as your hybrid does in the first few centimeters of the brake pedal's travel. In a Tesla, you hardly ever have to touch the brake pedal. But on your hybrid's system (and Ford Focus EV, I think) you don't know when you've reaching the max regeneration and the brake pads start scrubbing off energy and wasting it, because presumably they have designed it to be a smooth transition. Every time you inadvertently press the brakes a little harder than necessary, you lose efficiency. That can't happen in a Tesla.

@jat - of course it is not 100% single pedal driving. Did you think they put in a brake pedal for decoration??

The MS approach is simple to put regen as part of the right pedal vs the Prius and other approach of putting regen as part of the left pedal.

I drive both and my personal preference is the Tesla approach.

To be honest, I like this for another reason, safety. On the highway I had to brake hard several times. The nice thing with Tesla is when you take your foot off the accelerator and move your foot to the brake, your car has already started to dramatically slow down, even before you start braking. That extra second of breaking even before you push down on the break could be the difference between an accident or not.

"One other factor that may be a consideration is the case when the battery is full."

Hmm, that's an interesting point. When the battery is full, where does the regen energy go? Diesel locomotives have huge resistor packs on their roofs to dump energy as heat during regenerative braking. Where does it go in the Tesla?

@avanti - when the battery is full, regen braking is limited. People have complained that they expect the regen braking and are surprised that they don't slow down as much.

So in the model x, if your foot fully off the accelerator is the maximum regen, I presume the regen breaking will be twice as strong on the 4wd versions as the 2wd version.

Similarly the model S engine and regen might get stronger in future releases.

Would a stronger regen on the accelerator peddle be an issue at some point, that would require the strongest regen to link to the brake, or is it simply not an issue.

(Ps. I realise the 2wd model x will probably have less regen than the model S, with 4wd being similar to model S, just trying to explore the issues)

Why use your feet at all over 20mph. The cruise is well designed and intuitive.

Brakes are for emergencies only.

Another issue not mentioned above, is that necessarily regen "drags" on the rears only. Braking affects all 4, and the fronts mostly (inertia vectors there when slowing).

grega - incorrect assumption because at max regen, the Model S already puts as much energy back into the battery that it can accept. Unless the battery and charging technology changes, I would expect regen to feel pretty much the same on future TM cars.

It is a strange feeling to have regen limited when we max charge (every couple weeks we have to max charge due to a longer trip requirement) and the car behaves differently. I have run up toward the rear of a few cars when the car did not slow as it normally does when lifting off the accelerator. I wonder if there may eventually be a suit about this.

For now the car puts a yellow line on the battery charging indicator dial to show that regen is limited, but my opinion is that a car should have consistent brake feel and behavior for safety reasons. We therefore hope that TM finds a way to make it feel like there is regen happening even when the battery is already full.

I suppose the software could apply the brake in case of a full battery. But that assumes that the brake is drive by wire, and I don't think it is in the Model S.

@Pungoteague_Dave - If you want regen to be available all the time, the simple solution is don't charge to the max. A 99% charge should be enough to ensure regen is available all the time.

I've heard regen may also be limited when the battery pack is very cold, as the batteries are not in a good state to take a quick charge until they warm up. I haven't this myself, but I've never been in freezing temperatures either.

"I suppose the software could apply the brake in case of a full battery. But that assumes that the brake is drive by wire, and I don't think it is in the Model S."

You don't need drive-by-wire brakes to apply the brakes automatically. Lots of cars do that. Any car with adaptive cruise control can do it, and it is also needed for electronic stability control. I assume it is done with solenoid-driven actuators. The following is from the Tesla Model S website:

"Stability Control reacts in moments of under-steer or over-steer by reducing torque and applying the brakes to individual wheels for enhanced control when cornering."

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